Wave signal receiving system



Aug- 1l, 1936. H, A. WHEELER I* WAVE SIGNAL RECEIVING SYSTEM Filed Feb.2T, 1955 4 Sheets-Sheet 1 Aug. 1l, 1936.' v H A, WHEELER 2,050,680

wAvE SIGNAL RECEIVING SYSTEM Filed Feb. 27, 1935 4 Sheets-Sheet 4Patented Aug. ll, .1936

UNITED STATESv WAVE SIGNAL'RECEIVING SYSTEM Harold A. Wheeler, GreatNeck, N. Y., assignor to Hazeltine Corporation, a corporation ofDelaware Application February 27, v1935, serial No. 8,467 f 16 claims.

This invention relates to signal-modulated carrier wave signalingsystems, and more particularly to such systems designed fordistortionless and selective reception of radio signals, free frominterfering signals and other disturbances.

In my copending application, Serial No. 691,927, led October 3, 1933, ofwhich the present application is a continuation-in-part, there isdescribed a system for receiving signal-modulating carrier waves andutilizing only the carrier and either sideband of modulation. Asexplained at length in that application, such a system has a number ofadvantages, among which maybe noted that the signal selecting circuitsneed be designed-to pass only one-half the band of frequencies requiredby a conventional receiver, resulting in increased selectivity and aminimum of interference. Further, the system may beadjusted to `receivethat sideband which is subject to a minimum interference from signals onadjacent channels.

. The system of my aforesaid application included as desirable featuresthereof, provision for adjusting the circuits so that the signal carrieris located at either one edge or the other of the narrow frequency bandpassed by the selecting circuits; an arrangement of selectiveadmissioneffective to increase the response of a receiver when it is properlytuned to receive-a single sideband; an automatic volume control of thesuspended type, that is, one which operates to reduce the ampliilcationof the system only for received signals above a predetermined intensity;a reverse automatic volume control associated `with themain volumecontrol, procuring the required ultimate variation of the bias voltagefor a gain control of the system without any appreciable change intheampliedcarrier output; and-a quieting system leffective to maintain the"system unresponsive unless the received signal exceeds a predeterminedintensity and unless the receiver is correctly tuned.

My present invention relates to signal translating, amplifying andcontrolling circuits and sysl5-tems which, while of general application,are particularly suitable for use in connection with signal-modulatedcarrier wave systems of the type disclosed and claimed in my, aforesaidcopending application and. briefly described above. In- -cluded in mypresent invention is an arrangement for optionally operating a singlesideband 4 type otreeiving system for double sideband reception, whichmay be found desirable when a very weak signal ismbeing received and acondi- I05 'udn of vel-y mgn selectivity vmusi; be provided,

(Ci. Z50-20) The above and other features of the invention will becomeapparent from the following detailed description and reference to theaccompanying drawings.

In the accompanying drawings, Fig, 1 is a dia- 5 gram showing ingeneralized form a receiving system of the superheterodyne type, adaptedfor single sideband operation in accordance with this invention; Fig. 2is a schematic circuit diagram of a receiver constructed in accordancewith the 10 general diagram of Fig. 1; Fig. 3shows graphically theselectivity of the intermediate-frequency amplifier of Fig. 2; Fig. 4shows graphically the fidelity of the audio amplifier `of Fig.Zand-illustrates the compensation for the loss of part. of 15 a sidebandunder the condition of single sideband reception; Fig. 5 shows thechange of audio-frequency fidelity upon change of volume level; Fig. 6aillustrates the responsiveness of the intermediate-frequency trap ofFig. 2; Fig. 6b illustrates 20 the relationship between grid bias andplate current of the quieting tube; Fig. 6c illustrates theamplification characteristic of the '.uieting system; Figs. 6d and 6eillustrate, respectively, the variations in plate current and inrelative ampliiication of the quieting system; and Fig. 7 shows thechange of automatic volume control bias voltage with change ofintermediate-frequency carrier voltage, in the receiver of Fig. 2.

The general principles of single sideband reception and theirapplication to a superheterodyne radi receiver are discussed at lengthin my aforesaid copending application, to which reference is made'. Inbrief, the signal-selecting ciri cuits for transmitting thesignal-modulated carrier frequency are designed and adjusted to passonly the carrier, one sideband, and the inner part of the othersideband, so as to include the complete audio-frequency spectrum witheach audiofrequency component represented in its proper 40 relativemagnitude. In case of a receiver having a tuned radio-frequencyamplifier, but not the superheterodyne feature. the general principlesof operation are otherwise the same as in a conventional double sidebandreceiver, provision vbeing made for compensating for the loss of signalintensity by selecting against the outer part of the other sideband. Itis there explained, also, that in the case of superheterodyne radioreceivers, to which thevpresent invention is illustrated as applied, onesideband of the signalmodulated carrier frequency, when modulated in afrequency-changing circuit by the locally generated oscillations. whosefrequency is greater than the received carrier frequency, results in asingle sideband of difference frequency or intermediate frequencyshifted along the frequency scale but having characteristics in allother respects the same as the original sideband, save only that thehigh and low sidebands are reversed in position on the frequency scale.The amplification and detection of the intermediate-frequency modulatedcarrier through the remaining portions of the receiving system areaccomplished in the same general manner as in a conventional receiver.However, certain features described briefly above may be utilized toadvantage in a single sideb'and system of my invention to improve theselectivity, sensitivity and fidelity `of reproduction of the system.

Circuit arrangement and general operation Fig. l is a generalizedcircuit diagram illustrating the arrangement of 'a superheterodynereceiver adapted for single sdeband operation in accordance with thisinvention. The path traversed by signals between the antenna and theloudspeaker is shown conventionally. There are provided an antenna 30and ground 3i for intercepting the signals, which are then supplied, inorder, to a radio-frequency amplifier 32; a local oscillator andmodulator 33; an intermediatefrequency amplifier 34; a diode rectifier35; an

audio-frequency quieting amplifier 36; an audio-frequency amplifier 31;and a loudspeaker 38.

With reference to thisxnain signal path, the radio-frequency signal isreceived in the usual manner by the antenna 30 and selected andamplifled at the signal frequency in the radio-frequency amplifier 32,which for broadcast reception is capable of tuning over the frequencyrange of about 550 to 1500 kilocycles. The oscillator and modulatorsystem 33 converts the radio-carrier frequency signal to anintermediate-carrier frequency signal in the manner well understood inthe art. The intermediate-carrier frequency willbe one of twofrequencies depending upon which of the two sidebandsis being selected.These two alternative intermediate-carrier frequencies will, therefore,differ by the width of a sideband, which may be in the neighborhood of 3kilocycles. These intermediate-carrier frequencies may conveniently betaken as 173.5 or 176.5 kilocycles, depending upon which sideband is tobe utilized. 'Ihe signal is' further amplified in theintermediate-carrier frequency amplifier wherein the carrier and onesideband are selected, the band width passed by this amplifier being,for the particular intermediate-carrier frequencies mentioned above,173.5 to 176.5 kilocycles, centered at 175 kilocycles, The dioderectifier 3S derives from the carrier and the singleintermediate-frequency sideband the audio frequencies of modulation,which are then amplined by the audio amplifiers 3l and 31, from whencethey are supplied t the loudspeaker Il.

For the purpose of maintaining the volume output o! the receiversubstantially constant in spite of wide variations of received signalintensities, there is provided an automatic volume control systemcomprising connections 4l from the output terminals ofintermediate-frequency amplifier 34 to the following elements connectedin succession: an intermediate-frequency trap 4l, anintermediate-frequency amplifier 42, a diode rectifier 43 and a diodesuspender 4 4. The functions oi' the intermediate-frequency trap 4| andof the diode suspender '44 are described below. In accordance with theoperation of the automatic volume control system, there is created atthe diode suspender 44 a unidirectional voltage which varies withY thereceived signal intensity; and this unidirectional voltage is appliedthrough the automatic volume control 4bias connection 4l to the controlelements of amplifiers 32 and 34 5 and oscillator-modulator 33.

For the purpose of insuring the proper variation of control bias voltageover a wide range of received signal intensities there is provided aconnection 46 from the intermediate-frequency i0 amplifier 34 of themain signal path to a control element of the intermediate-frequencyamplifier 42 in the automatic volume control system. 'I'his is a reversecontrol bias connection, the function and operation of which are morefully del scribedv below.

In accordance with a feature of the invention, there is created what ishere termed selective admission, which is an operating characteristicwhereby the user is constrained to tune the receiver so that theintermediate-carrier frequency is located at either edge of the selectedintermediate-frequency band, that is, 173.5 or 176.5 kilocycles, in thiscase. The feature of selective admission is effected by the action ofthe intermediate-frequency trap 4i of the automatic volume controlsystem, and is perfected by the cooperation of the quieting systemassociated with amplifier 36, as will more fully appear below. The trap4i contains a selective circuit propor- 30 tioned to cause the automaticvolume control bias of conductor 45 to become partially relaxed when thereceiver is so tuned that the intermediatecarrier frequency is locatedexactly at the center of the selected intermediate-frequency band, that3 is, 175 kilocycles in this case. At this tuning point, theintermediate-frequency carrier voltage present at connections 40 ismaintained at a substantially higher level than in the case of any othertuning points.

There is associated with the audio-frequency amplifier 36 a quietingsystem whereby sideband selection and quieting between stations areobtained by the manner in which the amplifier is operated. To securethis operation, the amplifier tube is so adjusted that its gain is amaximum at a critical negative value of grid bias, and rapidlyapproaches zero for greater or lesser biasing voltages.

The-biasing voltage for the quieting tube is obtained from the dioderectifier and is proportional to the carrier voltage at connections 40.When no signal is present, or when the average rectified signal or noisevoltage is less than the critical bias, the gain through the quietingamplifier 36 is substantially zero, and there is practically notransmission through this amplifier. Also. when a strongintermediate-frequency carrier signal is tuned to the center of the band(175 kilocycles in this case), the bias developed by the rectifier 3Sbecomes considerably greater than the critical value, and the gain ofamplifier 3l is again reduced nearly to zero. The critical bias, whichproduces this operative condition, is obtained only when' theintermediate-frequency carrier is detuned'to either edge of the selectedband (173.5 or 176.5 kilocycles in this case). It should be noted thatthe grid bias is determined by the amplitude of carrier voltage atconnections 4U. This amplitude is in turn determined by the responsecharacteristic of the intermediate-frequency discriminating circuit, ortrap 4i, in cooperation with the automatic volume control system. Thetrap should, therefore, be made to have such a characteristic that thecritical bias 75 is obtained at either edge of the intermediatebe tunedas accurately fas those of the interfrequency band.

Fig. 2 is a detailed circuit diagram illustrating a superheterodynereceiver adapted for single sideband operation, designed according tothe general arrangement of Fig. 1. 'Ihe rectangular boxes of Fig. 1 areindicated in Fig. 2 in dotted lines and are similarly numbered.

In Fig. 2, the radio-frequency amplifier 32 comprises a pentode tube 6|suitably coupled to the antenna 38, and Vhas vassociated therewith threesimultaneously tunable selecting circuits, two of which, designated |30and |3|, are locatedahead of the amplier, and the other of which, |32,is coupled between the amplifier and the tube 62. These circuits aretuned by variable condensers |36, |36 and |31, respectively.y

The oscillator vand modulator arrangement 33 comprises the hexodeoscillator-modulator tube 62. Such an oscillator-modulator circuitconstitutes no part of this invention and consequently is not describedin detail here. This type of i' oscillator-modulator is described in myPatent No. 1,958,027, issued May 8, 19,34.' 'I'his oscillatormodulatorcomprises a frequency-determining circuit 99 having a variable tuningcondenser 2 I6.

The tuning condensers |86, |36, |31 and 2|6 are ganged together by amechanical.unicontrol deviceindicated by the dashed lines U. I'hesecondensers constitute the tuning means for tuning the signals to thedesired intermediate-frequency band.

The output of the oscillator-modulator is coupled to the input of theintermediate-frequency amplifier 34 which comprises two amplifying tubes53 and 64 and the three intermediate-fr@ quency coupling systems |60;|6| and |62 located before, between, and after, the amplifying tubes.

The signals are rectified in the diode portion of a double-diode pentodetube 66' in stage 36. Only one of the diodes is employed for signaldetection,

lthis particular diode comprising the cathode |63 and one of the diodeanodes |64. Thepentodel elements oi this tube are used in theaudio-frequency quieting amplifier 36. For the purpose of illustration,therefore, tube 66' is shown again in the audio amplifier section 36,where it is designated as tube 65. This second illustration of the samephysical elements is proper and convenient'because, insofar ascircuit'operation is concerned, the action is that of ,two separate anddistinct tubes. j

The rectined current flows through a resistor 83, from whence rectiedvoltage is impresscd'upon the control grid of the pento'de amplifier 66by way of conductor |66 through resistors 34, 81 vand 66 .and condensers86, 89 and 36. The resistor 81 and the condenser 89 are proportioned toproyvide proper audio-frequency compensation, as

will be more fully described hereinafter.

The anode of pentode amplifier 66 is coupled to A .amplified in thepush-pull amplifier including tubes 61 and 66, whence they arf` conveyedto the loudspeaker 38. y

It ls of primary importance that the receiving system select aband ofVfrequencies having a width about equal to the highest required 'audiofrequency of-modulation. Since the selective circuits of theaudio-frequency amplifier 32 cannotY mediate-frequency amplifier 34, itis preferable that they be tuned sufliciently broadly to pass a lbandofthis width witha uniformity within one tem |6| comprising adouble-tuned transformer figure the intermediate-frequency spectrum of asignal is represented at 26, being centered at frequency fie of 173.5kilocycles and having upper and lower intermediate-frequency sidebandswith boundaries fie', fin", respectively (corresponding to the lower andupper carrier sidebands, re-

spectively). In this same figure the curve ID represents the .selectioncharacteristics of the intermediate-frequency selecting circuits of-amplifier 34 centered at a frequency fie of 175 kilocycles and fiatwithin one decibel over a band o! -three kilocyclesiv that is, over theupper intermediate-frequency sideband. There is also shown in thisgurethe intermediate-frequency spectrum of two other signals and 2,correspond- 'ing to adjacent broadcast channels, and centered S5 at,fm-.10 and fie-F10 kilocycles, respectively; that is, separated by 10kilocycles from the signal 26 being received.

For Athepurpose of this example, it is Vassumed that signalilll isstronger than signal H2; hence,

the receiver isshown to be properly tuned to the sideband which isfarthest away from the stronger signal. 'I'he intermediate-carrierfrequency of signal and also its sideband nearest'to signal 26, aresubject to |6 decibels` more attenuation than the carrier frequency andnearest sideband of the signal ||2. Both of the adjacent channel signalsare attenuated much more than theyv would be if the curve |-|0 weredoubled in width to include both sidebands;v and it is shown how thereceiver may be tuned to discriminate against countered Ain the use oithis receiver for single sideband reception are: (1) to cause theoperator to tune the receiver correctly so that the inter- -the strongerof the signals in the two adjacent channels. v'Iwo of the most importantproblems enmediate-carrier frequency is located' at one edge of theselected frequency band. as shown in Fig. 3; and (2) to compensate forthe loss offnearly all of the rejected sideband.

' 'I'he iirst of these problems is solved by a system of selectiveadmission, which greatly lessens or entirely quiets the Vresponse of thereceiver when not correctly tuned. This system,

which has been mentioned above and is described below in more detail, ismade necessary or at least vdesirable because the sound at theloudspeaker is harsh and unpleasant when the carrier lfrequency is tunedtoo far oi! pf the edge of the transmitted sideband.

'I'he second of these problems isv solved by the' -use of resistor 31and condenser 38 associated with the audio-frequency tube 66. In Fig. 4.which is plotted as relative gain or responsiveness against 4 rent uponthe supply of intermediate-frequency audio frequency, curve ||3indicates the equivalent audio-frequency loss resulting from the cuttingout of most of one sideband. This loss is shown to be about 3 decibelsat 1 kilocycle and 6 decibels at 3 ki1ocyc1es,in the case of theparticular receiver under discussion. Accordingly, theelements 81 and 89are proportioned to produce a relative audio-frequency gain which variessubstantially as represented by curve I |4. This latter variation beingcomplementary to that of curve ||3, uniform overall audio-frequencyresponse is produced as represented by curve ||5. In producing thiscompensation it has been found proper to make the impedances of elements81 and. 89 of the same order of magnitude at one kilocycle.

Automatic volume control The proper operation of the quieting systemrequires that the output of the intermediatefrequency amplifier 34 bemaintained at a constant predetermined value, substantially independentof both the received signal intensity and the percentage of modulation.The arrangement and operation of the automaticvolume control system isdescribed with reference to Figs. l, 2 and 7. Intermediate-frequencyvoltage is derived from a coil 1| tuned by a con-l denser 12 and coupledto the primary coil of the intermediate-frequency coupling'system |62.and, by means of connection coupling transformers 14 and 61 of theintermediate-frequency trap 4|, and intermediate-frequency amplifier'tube 58, it is applied between the cathode 6| and diode anodes 60 and|12, constituting the rectifying portion of a double diode pentode tube59, hereinafterv called the A. V. C. diode since it determines theoperation of the automatic volume control. Y

Associated with the diode rectifier in the tube l59 is a voltage dividercomprising resistors 92 and 63 serially connected across a battery 68through the diode 55", constituting cathode |83 and diode anode |15 ofthe multi-purpose tube l5 referred to above, which is referred to hereinasthe suspended diode. The anodes 40 and |12 are connected to thejunction 84 of resistors 62 'and 63 which are so proportioned that, inthe absence of a. signal. the anodes are maintained substantially at thesame voltage with respect to ground as the cathode 8|, which is biasedwith respect to ground due to the flow of the space current of thepentode portion 0f the tube 59 through the resistor 55. The result isthat there is substantially no flow of current in the diode section ofthe tube 59, but it is in an incipient conductive condition and carriescurvoltage to the anodes 90 and |12. Under these conditions points 39and |01, which are connected to the grid of the pentode 59 and to theanode |15 of the diode 55", respectively,- are eii'ectively groundedthrough the diode 55." so

that no automatic volume control bias voltageis supplied to theconductor 45.

64. A further increase in signal intensity increases the negativevoltage at these points, which is applied through the conductors and 45to the control grids of the intermediate-frequency amplifying tubes 53and 54, the radiofrequency ampliiier tube 5|, and theoscillatormodulator tube 52, respectively.

The control grid |13 of the tube 59 is also effective under theseconditions to reduce the space current in this tube, and thus to reducethe voltage across the resistor 65. In this manner, the change ofnegative voltage on the cathode 5|, relative to ground, augments that ofthe anodes 60 and |12, with respect to the cathode 6|, therebyincreasing the resultant negative voltage existing at the point 64, andtherefore at the points 39 and |01.`

The operating characteristics of a typical automatic volume controlcircuit operating on these principles are shown in Fig. '1 of thedrawings, in which the line |23 represents the voltage of point 64 withrespect to ground, which voltage changes negatively in proportion to thesignal intensity; In this figure the dimension |24 indicates the voltageacross resistor 62 and the dimension |25 that across resistor 63. Whenthe signal intensity increases to the point where the voltage of thejunction point 64 becomes negative, .the current in the4 diode 55" isreduced to zero and the average voltage applied to the automatic volumecontrol bias conductors 45 and 45' from the points 39 and |01 becomesequal to that at the point 64.

Asstated above, a decrease in the voltage of the points 39 and |01decreases, by virtue of the connection of the grid |13 thereto, thespace current in the tube 59, and thus effectively increases the changeof negative voltage of the diode anodes 60 and |12, to eilect aresultant increase in the negative voltage of the point 64. This isrepresented by the increased slope 0f the curve |23 at its intersectionwith the zero axis. I f the effect of the pentode 59 were omitted, thecurve |23 would continue at its original slope, asr shown by the dottedline |21. In this diagram4 the ordinate |26 represents the voltage ofthe point |01 relative to ground.

The action of such automatic volume control biasing connections forcontrolling the gain of amplifiers andmodulators is well understood inthe-art and requires no detailed discussion here. It is sumcient tostate that the connections- 45 and 45 cause the control grids ofthelcontrolled tubes to become more negative when the signal strengthincreases above a predetermined value, thereby maintaining theintermediate-frequency output level fairly uniform. A

The desired highly uniform output of intermediate-frequency ampliiier 34is accomplished by means of a reverse automatic volume control biasvoltage provided by a connection 44 extending from the cathode ofamplifier tube I4 to the cathode of amplifier tube 58 of the automaticvolume control system. This c ommon cathode connection 46 is connectedto ground through a resistor 68 which carries the space current of bothtubes; hence, the space current of tube 54 partially controls thegrid-cathode bias of tube 58 and, therefore, the gain of this lattertube. 'I'his bias on tube 58, however, varies oppositely fromthat on thecontrolled amplifier and modulator tubes of the main signal path. Whenthe grid-cathode, bias of tube 54 becomes greater by automatic volumecontrol action, its ampliilcation and space current decrease, andconsequentsomewhat.

causing the gain of the latter tube to increase Therefore, the increasedintermediate-frequency voltage, which is required by the A. V. C. diodein tube 59 to decrease the amplification of amplifier 34, is supplied bythe f increased gain of tube and without any increase of the input tothe latter tube, cr of the output of amplifier 34. This action is calledreverse automatic volume control because the gain of tube 53 isautomatically varied oppositely to that of tubes 5I, 52, 53 and 54.Quantitatively, the gain of tube 58 can be just about doubled by thisreverse automatic volume control connection, this being suflicient tovary the intermediate-frequency voltage on the A. V. C. diode of tube 59over the desired wide range without any appreciable variation of theintermediate-frequency input to tube 58.

The reverse and the suspended .automatic volume control actionscooperate to maintain the output of intermediate-frequency amplifier 34very nearly uniform during reception of large variations of signalintensity.

Selective admission 4The selective admission, mentioned above, isprovided to cause the oscillator correctly `to tune the receiver forsingle sideband reception. This includes means for causing theaudio-frequency output to reach sharply a maximum value when theintermediate-carrier frequency is tuned on either edge of the bandselected in the intermediate-frequency amplifier 3 4.4 This action recase), so that this center frequency is sharply attenuated. Condenser l0is adjustable to en- .able the trap to be readily tuned to the centerfrequency. Coil "il is smaller than the primary coil of coupling system682 so as to produce a voltage step-down from the output of tube 54 tothe. trap circuit. Transformer 14, which couples the `trap to the inputof tube 58 is 'a step-up transformer in which the secondary coil fit isiarger than the primary coil 4S. A condenser '3% is inserted in serieswith the primary coil di@ of transformer i4 and serves to tune the saidprimary coil broadly to the center of the selected band (i75 kilocycles)The resistor 'il has the proper value to broaden the resonance cielements iii and l5; and the resistor 76 serves to limit the currentwhich enters the trap vfrom coil il. The ratios of the transformers atthe input and output of the trap are chosen to permit coil t8 to be ofthe same order of magnitude as the secondary coil 88 of transformer i6,in spite oi the fact that transformers y'i3 and lli work out of, andinto, respectively, circuits oi high impedance.

`'Zihe eifect of the automatic volume control system includingintermediate-frequency trap 4l upon the rectified output of the signalrectifier at tube is shown in Fig. 6a, in which curve il@ represents therectified signal voltage secured f when a signal at the input to section34 Ais tuned within the band uniformly passed by theintermediate-frequency selectors of section 34.- The sharp peak of curveH8 is caused by the action of the trap circuit in relaxing the A. V. C.voltage, hence increasing the amplification in section 34,

vwhen the carrier frequency is tuned to the trap frequency, 175kilocycles. The resulting signal level at the input of section 36reaches a sharp maximum as shown by curve H8 when the car'- 'I'he effectof the l rier is tuned to 175 kilocycles. trap characteristic H8, inenabling the receiver to be properly tuned, will vbe fully brought outin the following discussion of the quieting sys` tem.

Automatic queting The automatic quieting action is the means forquieting the receiver at all times except when the receiver is properlytuned at either edge of the intermediatefrequency band. This systemquiets undesired noises and distorted signals which would otherwise bereproduced when the receiver is not properly tuned to a useful signal.The quieting system cooperates with the trap 4i and the automatic volume'control system to produce the selective.admission.

The quieting actiontis obtained by reason of the peculiar transmission,or transfer ratio, characteristic which is imparted to the pentodetranslating tube 55. To produce the single sideband type of reception.the switches i9 and 8l, ganged for unitary operation, are thrown to thexight so that switch 80 connects a resistor al in series between thescreen 292 and the positive terminal of the direct voltage source 203.Resistor 8l has a relatively low value so that the voltage at screen 2ozis substantially independent of screen current. A relatively highresistor lo is con-y nected in series between the anode lll and thepositive side of direct voltage source 205, the latter source being inseries with source 203.

The relationship between the grid bias and the anode current of thispentode as thus connected, is shown by curve IIB of Fig. 6b,- whereingrid bias voltage is plotted against plate current in milliamperes.nearly to' zero by ananode current flow-of about l milliampere throughresistor 1l, the remaining space current of the tube flowing to thescreen 2&2. llt is observed from curve H6 that when the negative vgridbias reduces to about 9 volts, the plate current rises to about lmillampere. Hence, for values of negative grid bias less than 9 volts,

the plate voltage is substantially zero. For

values of negative grid bias greater than 9 volts',

The anode, voltage isreduced the total space current is reduced andtherefore the anode current falls rapidly, approaching zero beyond l5volts bias.

The gain, or amplification, ratio of quieting tube 55 depends on theslope of curve H6 and is, therefore, a maximum in the region between thelimiting values of 10 and l2 volts negative bias. The slope in thisregion represents an amplification of about nity times, for theparticular tube arrangement used. The relative amplification, orrepeating ratio, of the pentode bias controlled repeater tube 55isrepresented by curve lll of Fig. 6c, in which percentage amplificationis plotted against negativey grid bias. For the parseen to be a criticalvalue of grid bias, since only those bias voltages which are at and nearthis tisular tubearrangement described, 11 volts is Il. Condenser 02 isso small as to have a negligible effect at audio frequencies and serveswith resistor I4 and condenser 85 to remove the carrier-frequencycomponents from the rectified voltage. The lower audio-frequencymodulation components of the Vrectified voltage follow the path ofelements 86, 81 and 88 to the control grid |15 of pentode 55. Theimpedance ofthe input circuit of tube 55 to higher audio frequencies isdecreased by condenser I8 and a relatively low resistor 90, connectedacross the high resistor 01. This lower impedance path 89, 90compensates for the selection of the lower audio frequencies of bothsidebands. Grid bias voltage proportional to the average rectied signalvoltage is applied to control grid |16 through a path including the highresistors 8G, 8| and 92. Condenser 93 has a sufilciently large capacityto bypass all modulation components at this biasing connection.

The entire direct voltage appearing across resistor 83 is applied togrid |16 asla quieting bias. But only a small portion (about 1A3) of theaudiofrequency component of rectified voltage is applied to grid |16 asa signal. This limits the grid swing to a value which will not cause thepeaks oi the waves to lie beyond the straight portion of curve H6,thereby avoiding distortion.

The bias voltage derived from diode 55' varies in accordance with curveH0 of Fig. 6a. It is observed from curve H8 that the trap 0| whichproduces this characteristic is so proportioned as to cause the criticalbias voltage (ll volts in'Fig. 6b) to be obtained when the carrier istuned to either edge of the selected intermediate-frequency band (1.5kilocycles ony either side of the center in this case). Because of thisrelationship between curves ||6 and H8, pentode tube 55 is operative topass signals only when the intermediste-carrier frequency is properlytuned for single sideband reception at either-edge of the selected band.i

Visual tuning indicator There is connected in series Awith a resistor 18in the anode circuit of tube 55 a visual tuning indicator in the form ofa milliameter 98. When the receiver is in operation, this milliameterhas a maximum deflection of l milliampere and a minimum deilection ofzero, as can be observed from Fig. 6b. The milliameter indicates theproper tuning position for single sideband operation when th'e meterreads half deection at 0.5 milliampere. Fig. 6b shows this to be theanode current which flows when the critical grid bias voltage of 11volts is reached. Curve lli oi' Fig. 6d and curve |20 of Fig. 6e show,respectively, the variations" in anode current and in relativeamplification in tube 55 when the intermediatecarrier frequency is tunedin therreglon of 175 kilocycles. The two sharp peaks of curve |20 showthe audibility points of correct tuning of the intermediate-carrierfrequency at 1.5 kilocycles above or below the mid-point frequency oi.175 kilocycles, and correspond to mid-scale deilection of thetuning,meter.

Double sideband reception To cause the receiver to operate as a doublesideband receiver, switches 10 and 80 are thrown to the left position.Switch then causes resistor 8| to become open-circuated so that thescreen voltage is furnished from the positive side of voltage source 205through a relatively high resistor 05. Switch l0. also then closes aconnection, placing in the grid bias circuit a resistory 91 whichreduces the bias voltage to a lower value than that used for the singlesideband adjustment. A bias voltagewhich produces a 0.5 milliampereanode current when a signal is tuned to 175 kilocycles has been foundcorrect. The average screen voltage and screen current are, under thiscondition, held nearly constant for small values of grid bias. Thisresults in an amplifier plate current-grid bias curve of normal shape,in which the slope is always positive.

The switch 19, in the left position, now shortcircuits resistor 95 whichreduces the audiofrequency voltage applied to the control grid |16. Atthe same time, this provides a lower impedance path to ground for thehigher audio frequencies through resistor 94. The increasedamplification of the high audio frequencies (under single sidebandreception) is thus removed, since high audio-frequency compensation isnot required under double sideband reception; Resistors @t and 95 areproportioned to cause the audiofrequency voltage at the anode |-11 to bethe same for both single and double sideband reception.

Under this condition of double sideband reception, the receiver is tunedto a minimum deilection on the tuning meter which will be at aboutmid-scale. This corresponds to tuning so that the intermediate-carrierfrequency is exactly at the center of the intermediate-frequencyselected band kilocycles) and is indicated by the loudest signal.

In the single sideband condition of operation, all signals of intensitygreater than 'a threshold value .are received at two points on thetuning dial corresponding to one or the other of the two sidebands.Signals of greater or lower intensity than the threshold value can bereceived by throwing switches 19 and 80 to the double sideband position.The signal is then received at only one point corresponding tocenter-tuning and highly selective double sideband reception. The higheraudio-frequency response, and consequently the noise which attendssignals of low intensity, are thus, reduced by this highly selectivedouble sideband reception.

It is a feature of the present invention that with switches 19, 80 inthe position for single aideband reception, signals of lower thanthreshold intensity are received automatically at only one pointcorresponding to 'center tuning and double sideband reception.

The transition from the single sideband to the double sideband operationtakes place automatically when the signal intensity drops only a certainamount below the threshold value. The reason for this transition can beobserved from an inspection of curve |08 of Fig. 6a. This curverepresents rectified voltage applied as a. bias to the grid of thequieting tube when the signal tuned in is just strong enough so that thepeel: reaches the critical value of 1l volts. Under this condition. thesignal is tuned in at the point on the dial corresponding to 175kilocycles.

oscillator-modulator Briefly, the oscillatory system is associated withthe inner electrodes 201, 209 and 209. A

feedback coil 213 is connected in series withv a.

choke coil 211v and a direct voltage source 219, between the innerscreen, or oscillator anode 209 and ground. Coil 219 is coupled to acoil 214 included in the oscillatory system. The oscillatorfrequency-determining circuit 99 is composed of coil 214, a variablecondenser 215 and a fixed (though adjustable) condenser 216. .A couplingcondenser 219 is connected from the junction of elements 213 and 211 tothe junction of elements 214 and 216. Hence, there is capacitive as wellas magnetic feedback coupling from electrode'.

209 to the oscillator circuit 99. This arrangement vof the oscillatorysystem lprovides a uniform oscillator voltage over the entire frequencyrange of the oscillator.

` audiov range.

The oscillatory circuit 99 is also coupled both capacitively andmagnetically to the circuit of grid 208. The magnetic coupling existsbetween coil 214 and a coil 220 of the grid-cathode circuit; and thecapacitive coupling is due to a deadend'turn 221- connected with coil220 and elec-v trostatically associated'with coil 214. By reason of thisarrangement, any capacity change in the circuit of grid 2&9 produces afrequency change which is very small and substantially the same for alloscillator frequencies,

The coupling elements 213, 219, 220 and 221, when adjusted to the propervalues, remain permanently xecl at such desired values.

Volume level'control and ione compensation The volume level control andtone compensator comprises the adjustable voltage divider 100 at theinput of audio amplifier tube 56. The voltage divider is designed toprovide an approximately exponential relationship between the decibelsattenuation and 4angle of rotation of the contact control knob(represented by the arrow).

Experience teaches that the normal ear desires less change of intensityat the higher and lower audio-frequencies than at the mediu'n 'audiofrequencies. The volume level control is designed to satisfy this desireof the ear. At the greatest volume, the contact of potentiometer 100islocated at the upper end thereof. In this position, the elementsassociated with the potentivolume level, the fidelity curve issubstantially dat as shown by curve 109. In this potentiometerarrangement, elements 101 and 102 are small equal condenserswhich reducethe attenuation changes at the higher audio frequencies; and elements104 and 105 are large equal condensers which reduce the attenuationchanges at the lower audio frequencies.

It should be understood that the foregoing description applies to apreierredembodiment of '.the inventions and should not be construed asa.

limitation thereof. For example, values other than those used herein, ofvoltage, grid bias, frequency and band width, may be employed. Likewise,vacuum tubes of equivalent performance but having more or lesselectrodes per tube may be substituted. Although, in the drawings, thedirect voltage sources are, for convenience, shown symbolically -in theform of separate batteries, it should be understood -that a commonsource of power supply of any of the conventional types may be usedrather than separate sources in the several locations where batteriesare represented.

What is claimed is: l. In a modulated-carrier-signal receiver, a rectiercircuit for rectifying the signal to produce a, modulated unidirectionalvoltage, a biascontrolled modulation voltage repeater stage having inputand output circuits, means for applying at least a part of saidunidirectional voltage to said input circuit as bias and inputmodulation voltage, and means for causing the transfer ratio of saidstage to be maximum for a predetermined value of bias voltage and toapproach closely zero for all substantially different values, wherebythe output modulation voltage is caused to be substantial only when thesignal causes said bias voltage to be substantially equal to saidpre- Qi) determined value.

2. In a modulated-carrier-signal receiver, a' rectiner circuit forrectifying the signal to produce a modulated unidirectional voltage, amodulation-voltage repeater stage including a pentode -o repeater havinga cathode, control grid, screen,

ysuppressor and anode situated in the order named, 'means for impressingsaid unidirectional voltage on said grid .as the modulation-voltageinput and as a bias voltage negative relative to said cathode, means forimpressing on said screen a substantially constant direct voltagepositive relative to said cathode, means for maintaining said suppressorat subtantially zero voltage relative to said cathode, means forimpressing on said anode ay direct Avoltage from' a source positiverelative to said cathode, and aresistor between vsaid anode and saidsource for reducing the `anode direct voltage to approximately halt thesource -voltage for a substantial negative value of said bias voltage,whereby the modulation-voltage output oi' said repeater stage is causedto be sharply maximum for the value of modulationvoltage inputcorresponding to said value oi' bias voltage.

3.*In a modulated-carrier-signal receiver, a-

rectifier circuit for rectifying the signal to pro- Y duce amodulated'unidirectional voltagefa modulation-voltage repeater stageincluding a pentode repeater having a cathode, control grid, screen,suppressor and 4anode situated in the order named,.means for impressingsaid unidirectional voltage on said grid as the modulation-voltage inputand as a 'bias voltage negative relative to said cathode, means forimpressing on said screen a substantially constant direct voltagepositive relative to said cathode, means for maintaining said suppressorat substantially zero voltage relative to said cathode, means forimpressing on said anode a direct voltage from a source positiverelative to said cathode, and a resistor between said anode and saidsource for reducing the anode direct voltage to a value not exceedinghalf the source voltage for a substantial negative value of said biasvoltage, whereby the amplification in said repeater is caused toincrease substantially with increasing input amplitude for inputamplitudes corresponding to voltages less than said substantial value.

4. In a modulated-carrier-signal receiver including a carrier-frequencyampliler, a rectien TID and a modulation-frequency amplier havingcathode, control grid, screen and anode, means for automaticallyregulating the output voltage of said carrier-frequency amplier, meansfor coupling said output voltage to said rectifier for rectification,means for coupling to the gridcathode section of saidmodulation-frequency amplifier the resulting modulated unidirectionalvoltage, means forimpressing on said screen a substantially constantdirect voltage positive relative to cathode, and means for impressing onsaid anode through a. high resistor a direct voltage positive relativeto cathode, saidresistor and both said direct voltages beingproportioned to cause the modulation-frequency amplification to .besharply maximum at a predetermined value of grid-bias voltagesubstantially negative relative to cathode, whereby a predeterminedvalue of carrier-frequency output voltage is caused to produce saidgrid-bias voltage and thereby to produce maximum modulation-frequencyvoltage across said resistor.

5. In a modulated-carrier-signal receiver, a carrier-frequency amplifierincluding .a band selector, a control circuit including a controlrectifier and a,` trap circuit sharply tuned to the center frequency ofsaid band for coupling the output of said amplifier to said controlrectifier, means for reducing the amplification in said amplifler withincreasing rectified voltage from said control rectifier, a mainrectifier for rectifying the'output of said amplifier to produce amodulated unidirectional voltage, a modulation-frequency amplifier foramplifying the modulationfrequency component of said unidirectionalvoltage, means for impressing on said modulationfrequency amplifier abias voltage which varies in accordance with said unidirectionalfvoltage, said modulation-frequency amplifier having an amplificationratio which decreases abruptly as said bias voltage departs either wayfrom a predetermined value, said control circuit being proo portioned toprovide a rectified voltage of an amplitude which maintains the carrieroutput of said amplifier at a value which causes said bias voltage toassume said predetermined value when the carrier frequency equals eitherof the two frequenciesI differing from said center frequency by apredetermined difference not exceeding half the band width of said bandselector, and at a substantially greater value when the carrierfrequency equals said center frequency.

6. In a modulated-carrier-signal receiver, a band selector, acarrier-frequency amplifier, means for causing the output of saidreceiver to be maximum only at a predetermined criticalV value ofamplified carrier voltage, and auxiliary means for automaticallymaintaining the amplitude of said amplified carrier voltage nearlyindependent of the received signal intensity, said auxiliary meansincluding a sharply selective circuit tuned to the center frequency ofsaid band for causing said amplified carrier voltage'to exceedsubstantially said'critical value only whenthe carrier is tunedsubstantially to said center frequency, and said auxiliary means beingproportioned to maintain said amplified carrier voltage at said criticalvalue, and thereby to cause the output of said receiver to be maximum,only when the carrier frequency equals either of the two frequenciesdiffering from said center frequency by a predetermined difference notexceeding half the band width of said band selector.

7. In a modulated-carrier-signai receiver according to claim 6.adjustable means for enabling said sharply selective circuit to be tunedto said center frequency in the band of said selector, whereby thetwofrequencies of maximum output may be adjusted relative to said centerfrequency without altering the difference between said two 5frequencies, which differ from said center frequency by a predetermineddierence not exceeding half the band width of said band selector.

8. In a modulated-carrier-signal,- receiver, a band selector forselecting a frequency band equal in width to one of the modulationsidebands of a modulated-carrier signal, tunable means for adjusting thecarrier to any frequency within said band, means'for greatly attenuatingthe output of said system except when the carrier is adjusted to eithexedge of said band, and a switch for disabling said output-attenuatingmeans to permit the operation of said system when the carrier isadjusted to the center of said band.

9. In a. modulated-carrier-signal receiver, a 20 band selector forselecting a frequency band equal in width to one of the modulationsidebands of a modulated-carrier signal. tunable means for adjusting thecarrier frequency to any frequency within said band, selective automaticcontrol 25 means for causing input signals of an intensity greater thana predetermined threshold value to produce maximum output when thecarrier frequency is adjusted to either of the edges of said band, saidcontrol means being proportioned 30 automatically to cause signals of anintensity lesa than said threshold value to produce maximum output whenthe carrier frequency is adjusted to the' center of said band.

lo. In'amcdulated-carrer-signal receiver, a 35 band selector forselecting a frequency band equal in width to one of the modulationsidebands of a modulated-carriersignal, tunable means for adjusting`thecarrier frequency tovany frequency within said band, selective automaticcontrol means for causing signals of an intensity greater' than apredetermined threshold value to produce maximum output from saidreceiver when the carrier frequency is adjusted to either edge of saidband, said control means comprising auxiliary means for maintaining theoutput voltage of seid band selector substantially independent of theintensity of said signals above said threshold value, a rectifierproducing a unidirectional voltage dependenton said output voltage,means for impressing the modulation components of said unidirectionalvoltage on the control grid of an amplifier tube adjusted to amplifyonly at or near a critical value of grid bias, a connection from saidrectifier to said grid for controlling said 55 grid according to theaverage value of said unidirectional voltage, a trap circuit associatedwith said auxiliary means for causing said output voltage to reach asharp maximum when said carrier` frequency ls adjusted to the center ofsaid band, whereby said critical value of grid bias is substantiallyexceeded when said carrier frequency is so adjusted. and said criticalvalue is obtained only when the carrier frequency is adjusted to eitherof the edges of said band.

1l. In a modulated-carrier-signal receiver for receiving either one ofthe modulation sidebands of a modulated-carrier signal, a band selectorfor selecting a frequency band equal in width to one of said sidebands,tunable means for adjusting the carrier frequencyto any frequency withinsaid band, a tuning meter for indicating the space current of a vacuumtube in'said system, means for limiting said space current to a maximummeter deflection in the absence of a 75 signal, selective automaticcontrol meansl for reducing said space current to half maximumdeflection when said carrier frequency is adjusted to either of theedges of said band, and to a very small deflection when said carrierfrequency is adjusted to the center of said band.`

l2: In a wave signaling system designed lto transmit a band offrequencies comprising a carrier frequency and at least one modulationsideband, a signal-translating .stage comprising means for producing aunidirectional voltage which varies with variations in frequency,vwith-` in said band, of signal input to said stage, a bias-controlledrepeater having a repeating ratio which decreases abruptly as the biasdeparts either way 'from a predetermined value, and

means for applying said unidirectional voltage to said repeater as abias voltage whereby the amplitude of the signal output from saidrepeater is caused to be sharply maximum for a critical value offrequency,`within said band, of signal input. v

13. In a wave signaling system designed to transmit a band offrequencies comprising a carrier frequency and at least one modulationsideband, a signal-translating stage Acomprising means for producing aunidirectional voltagea selective circuit excited from'the signal inputof said stage and eective to vary the unidirectional A 14. In a wavesignaling vsystem designed tor transmit a band of frequencies comprisinga carrier frequency and at least one modulation sideband, asignal-translating stage4 comprising a rectifier for producing aunidirectional voltage proportional to the input carrier amplitude ofsaid stage, a selective circuit for varying the carrier amplitude oftheinput to said stage between predetermined minimum and maximum valueswith variations in frequency, within said band, of signal input to saidstage, a bias-controlled repeater having a repeating ratio whichdecreases abruptly as the bias departs either way from a predeterminedvalue, and means for applying said unidirectional voltage to saidrepeater as a bias voltage,v whereby the amplitude of the signal output`from said repeater is caused to be sharply maximum for a critical valueof 'frequency withinv said band -of the signal input to said stage.

15. In a wave signaling system designed to transmit va band offrequencies comprising a carrier frequency and at least one modulationsideband, a signal-translating f stage, control means for automaticallymaintaining the carrier amplitude of the input to said lstage within arelatively narrow range for a wide range of inp ut carrier amplitudes tosaid system, said stage comprising' a rectifier for producing aunidirectional voltage proportional to the input carrier amplitude ofsaid stage, a selective circuit for modifyingv the action of saidcontrol means t0 vary the carrier amplitude of the' input to said stagebetween predetermlnedminimum and maximum values with -variations infrequency, within said band, of signal input to said stage, abiascontrolled repeater having a repeating ratio which decreasesabruptly as the bias departs either way from a' predetermined value, andmeans for applying said unidirectional voltage to said repeater as abias voltage, whereby the amplitude of the signal .output from saidrepeater is caused to be sharply maximum for a critical valueoffrequency within said band of the signal network substantiallyindependent of the carrier 3 amplitude of the input thereto for 'a widerange of input carrier amplitudes on carrier frequencies sov within saidband, a rectifier coupled to the output of said network for producing aunidirectional voltage proportional to the carrier amplitude thereof,\aselective circuit for causing the out- -put carrier amplitude andtherefore said unidirectional voltage .to vary between predeterminedminimum and maximum values as the frequency' changer is tuned to changesignal carrier frequency to different frequencies within said band, anda bias-controlled signal repeater coupled to the out-.-4

put of said network and having applied thereto said unidirectionalvoltage as a bias voltage, the

circuit of said repeater beingproportioned to se-A cure maximumrepeating ratio for an intermediate value of said unidirectional voltageand greatly reduced repeating ratio for said minimumfand'

